A platelet aggregation-inducing factor Aggrus (also known as podoplanin, gp44, etc.), a type-I transmembrane sialoglycoprotein, has been shown to be upregulated in many cancers such as different types of squamous cell carcinoma, mesothelioma, Kaposi's sarcoma, testicular germ cell tumors, and brain tumors (Non-patent documents 1-9). In some reports, Aggrus overexpression was correlated with a poorer prognosis, suggesting the important contribution of Aggrus to cancer progression (Non-patent documents 10, 11). Expression of Aggrus induced platelet aggregation, and promoted both experimental and spontaneous pulmonary metastasis in mice (Non-patent documents 11, 12). The platelet aggregation-inducing activity of Aggrus is directly linked to metastasis formation because the introduction of a point mutation that suppressed platelet aggregation attenuated the formation of pulmonary metastasis (Non-patent documents 11, 12). Cancer cell-induced platelet aggregation is believed to form a large cancer cell-platelet aggregate, resulting in enhanced cancer cell embolization in the microvasculature and protection from immunological assault in the circulation. The C-type lectin-like receptor 2 (CLEC-2) expressed on platelets was recently identified as one of the counter-receptors of Aggrus. When CLEC-2 binds to Aggrus expressed on tumor cells, it generates activation signals in platelets with no requirement for plasma components. Aggrus's critical domains for the interaction to CLEC-2 have been identified (Non-patent document 13).
A monoclonal antibody (mAb) can bind firmly and specifically to cell surface antigens and can induce an immunological response in the target cell. Thus, many mAbs are now used in cancer therapy. The mAbs used in cancer therapy show anti-cancer effects via three representative modes of action: neutralizing, antibody-dependent cellular cytotoxic (ADCC), and complement-dependent cytotoxic (CDC) activities. The mAbs, including bevacizumab and cetuximab, could neutralize activation of signal pathways by inhibiting either ligand-receptor binding or receptor oligomerization. Because cancer proliferation depends on activation of signal pathways, neutralization of signal pathway induces tumor cell death. Moreover, the mAbs, including rituximab and trastuzumab, could also induce immune response to target cancer cells via their fragment crystallizable (Fc) region. Complements and effector cells such as macrophage, natural killer (NK), and neutrophil can kill target cancer cells by recognizing Fc region of mAbs that bind cancer specific antigens. These three modes of action are specified by the isotype and subclass of the antibody, the traits of the antigen, and the recognition site.
Many mAbs against Aggrus have been established so far, but most of them cannot interfere with Aggrus-CLEC-2 interaction. Although one rat mAb, designated as NZ-1, is known as the Aggrus mAb capable of inhibiting Aggrus-CLEC-2 interaction and platelet aggregation (Non-patent document 14), however it is a rat antibody and cannot be precisely examined in universally-used mouse cancer model because of the species barrier.